Exposure to asbestos has been definitely associated with development of lung cancer. Even small amounts of asbestos together with cigarette smoking, have been shown to have a synergistic effect in producing lung cancer. In a preliminary study, large numbers of asbestos bodies were found more frequently in lung cancer patients than in a control population without lung cancer. We propose to confirm this finding by quantitation of bodies by both light and electron microscopy and specific identification of body cores as asbestos by electron diffraction in 100 patients with lung cancer and 100 controls matched for age, sex, and smoking habits. It is hoped that quantitative criteria for "significant" exposure to asbestos can be determined, and the magnitude of the problem of increasing evidence of asbestos in the environment clarified.